1. Field of the Invention
The present invention relates to an electrophotographic apparatus, and more particularly to an electrophotographic apparatus with an improved light receiving member.
2. Related Background Art
There have been known many electrophotographic methods, for example, as described in U.S. Pat. No. 2,297,692, Japanese Patent Publication No. 42-23910, and Japanese Patent Publication No. 43-24748. It is common practice to utilize a light receiving member, form an electric latent image on the light receiving member by various means, then develop the latent image with a developing agent (developer), electrically transfer the developer image onto a transfer medium such as paper as occasion demands, and thereafter fix the image by heat, pressure, heat and pressure, or solvent vapor or the like to obtain a copy.
In the above steps, since the residual developer remains on the surface of the light receiving member even after the developer image has been transferred onto the transfer medium, a cleaning blade, used as a means for removing the residual developer, is put in contact with the surface of the light receiving member to scrape the residual developer therefrom and discharge the untransferred developer to the outside of the system.
As the materials for the light receiving member used as an electrophotographic photosensitive member, a variety of materials are proposed, including inorganic materials such as selenium, cadmium sulfide, zinc oxide, and amorphous silicon (hereinafter referred to as a-Si), organic materials, and so on. Of these materials, non-monocrystalline deposited films containing silicon atoms as a main component, typified by a-Si, for example amorphous deposited films of a-Si or the like containing hydrogen and/or halogen (for example, fluorine, chlorine, etc.) (for example, compensating for hydrogen or dangling bonds), are suggested as high-performance, high-durability, and nonpolluting photosensitive members and some of them are practically used. U.S. Pat. No. 4,265,991 discloses the technology of the electrophotographic photosensitive member the photoconductive layer of which is formed mainly of a-Si. Further, as techniques for enhancing water repellency and wear resistance, Japanese Patent Application Laid-Open No. 60-12554 (U.S. Pat. No. 4,559,289) discloses a surface layer containing carbon and halogen atoms in the surface of a photoconductive layer comprised of amorphous silicon containing silicon atoms, and Japanese Patent Application Laid-Open No. 2-111962 discloses a photosensitive member having a surface protecting-lubricating layer provided on an a-Si:H or a-C:H photosensitive layer. However, these publications include no description concerning the relationship between the electrophotographic process and the scraping property of the surface layer.
Since the a-Si base photosensitive members, typified by a-Si, have excellent properties that they demonstrate high sensitivity to light of long wavelengths such as semiconductor lasers (770 nm to 800 nm) and have little deterioration recognized after repetitive use, they are widely used as photosensitive members for electrophotography, for example, in high-speed copying machines, LBPs (laser beam printers), and so on.
As the methods for forming the silicon base non-monocrystalline deposited films, there are many known methods, including the sputtering method, the method of decomposing a source gas by heat (thermal CVD method), the method of decomposing a source gas by light (photo CVD method), the method of decomposing a source gas by plasma (plasma CVD method), and so on. Of these methods, the plasma CVD method, which is a method of decomposing a source gas by a glow discharge or the like generated by direct current, high frequency (RF or VHF), or microwave to form a deposited film on a desired substrate such as glass, quartz, a heat-resistant synthetic resin film, stainless steel, or aluminum are now under way to practical use, including not only the method of forming the amorphous silicon deposited films for electrophotography, but also methods for forming deposited films for the other uses, and there are also proposed various apparatuses for such methods.
For the light receiving members, there are recently required improvement in the electrophotographic characteristics matching with high-speed operation and vivider image quality. Therefore, in addition to the improvement in the characteristics of the photosensitive member, the grain diameters of the developer are being decreased and there are frequently used those developers having the weight average grain diameter of 5 to 8 .mu.m measured by a coulter counter or the like.
As the charging and decharging means for the conventional light receiving members including the a-Si type light receiving member, there has been utilized in most cases the corona charger (corotron, scorotron) containing a wire electrode (a metal wire such as a gold plated tungsten wire of 50 to 100 .mu.m.phi.) and a shield plate as main components. That is, the charging and decharging of the light receiving member using the corona charger is carried out by applying a high voltage (about 4 to 8 kV) to the wire electrode to generate a corona current and allowing the corona current to act on the light receiving member. The corona charger is excellent in uniform charging and decharging.
However, the corona discharge is accompanied by generation of ozone (O.sub.3). The ozone oxidizes nitrogen in the air to form nitrogen oxides (NOx). Further, the nitrogen oxides react with water in the air to form nitric acid and other products.
The products due to the corona discharge such as the nitrogen oxides, nitric acid, etc., adhere to and are deposited on the surface of the light receiving member and peripheral devices. Since the corona discharge products have a strong hygroscopic property, deposition of the corona discharge products on the surface of the light receiving member results in reduction of the resistance of the surface due to moisture absorption of the corona discharge products to substantially decrease the charge retaining capability of the light receiving member throughout or in part of the surface, which may cause the image defect called image smearing (the charge in the surface of the light receiving member leaks in the plane directions to destroy or fail to form an electrostatic latent image pattern).
Further, the corona discharge products adhering to the internal surface of a shield plate of the corona charger are evaporated and liberated not only during operation of the electrophotographic apparatus but also during quiescent periods of the apparatus, e.g. during the nighttime, and they then adhere to the surface of the light receiving member at a part thereof corresponding to the discharge aperture region of the charger and absorb moisture to decrease the resistance of the surface of the light receiving member.
As a result, it becomes easier to cause the image smearing in the first image or subsequent several images outputted when restarting the operation of the electrophotographic apparatus, at the region corresponding to the aperture portion of the charger.
Further, the a-Si type light receiving member has a surface hardness extremely higher than those of the other light receiving members. Therefore, the corona discharge product adhering to the surface of the light receiving member can not be removed by the ordinary cleaning step of the light receiving member surface, so that the corona discharge product is likely to remain on the light receiving member surface.
Thus, hitherto, it has been sometimes practiced to provide a heater for directly heating the light receiving member or to send hot air to the light receiving member by a hot air sending device to heat the light receiving member surface (at 30 to 50.degree. C.) to thereby maintain the dry state, thus preventing the corona discharge products adhering to the light receiving member surface from absorbing moisture to substantially lower the resistance of the light receiving member surface and preventing the image smearing phenomenon from occurring. Most of the electrophotographic apparatuses using the a-Si type light receiving member have a heating/drying means incorporated therein.
Incidentally, the electrophotographic apparatuses are sometimes provided with a rotating cylindrical developer-carrying member containing a movable magnet or the like therein. In this case, there is widely used the method of forming on the carrying member a thin layer of a toner as the developer or a mixture of a toner and a carrier and then electrostatically transferring the toner onto a light receiving member having an electrostatic latent image formed thereon. For example, Japanese Patent Application Laid-Open Nos. 54-43037, 58-144865 and 60-7451 disclose the above method in which a developer such as a toner containing magnetic particles, i.e., a mixture of a toner and a carrier, or a toner containing magnetite but containing no carrier, or the like is used.
In such a developing method, there is a case where a portion of the rotating cylindrical developer-carrying member which is in opposition to the light receiving member expands by the heat radiated by the light receiving member during quiescent periods of the electrophotographic apparatus, so that the distance between the rotating cylindrical developer-carrying member and the light receiving member at the by the developer developing portion becomes short.
Reduction of the distance between the rotating cylindrical developer-carrying member and the light receiving member increases the electric field applied thereto to thereby allow the developer to be transferred more easily. This affects a portion at the side opposite the above mentioned portion to increase the distance between the above mentioned members to thereby decrease the electric field applied thereto, whereby the developer can be transferred with difficulty than usual. As a result, there is sometimes caused a problem of partial image density irregularity or the like at the period of rotation of the rotating cylindrical developer-carrying member. In order to obviate such phenomena, there is a need for an electrophotographic apparatus that causes no image smearing even when the light receiving member is not heated.
Further, with an electrophotographic apparatus in which the steps of charging, exposure, developing, transfer, separation, and cleaning are successively repeated and scrape cleaning with a blade is carried out, there is a case where the repeating operation gradually increases the wear resistance of the light receiving member surface. The increase of the wear resistance of the light receiving member surface promotes the degradation of the cleaning blade to lower the cleaning property for the remaining developer (hereinafter, referred to as "remaining toner").
When the copying step is repeated in this state, fine particles of the developer and additives (strontium titanate, silica, etc.) contained in the developer may be scattered in a corona charger to adhere to a wire electrode of the corona charger (hereinafter referred to as a charger wire), thereby causing discharge irregularities. When the discharge irregularities due to the contamination of the charger wire are caused, in the case of the positive development method (a method of developing unexposed portions of the surface of the light receiving member), image defects such as linear blank area portions on the image, scale-like black fogs spreading over the entirety of the image, local black dots (0.1 to 0.3 mm.phi.) without periodicity, and so on may be caused.
Further, when the contamination of the charger wire is caused, abnormal discharge may be induced between the contaminated portion of the wire and the light receiving member, thus damaging the surface of the photosensitive member to cause white dot like image defects.
Further, in such a blade type cleaning method, there is a case where differences are made among amounts of the developer staying on the blade surface because of differences in character patterns in an original chart and uneven scraping may occur in the surface layer of the light receiving member. When such uneven scraping occurs, sensitivity irregularities appear as electrophotographic characteristics and result in density irregularities in an image. This phenomenon becomes more prominent particularly as the grain diameters of the developer decrease.
In recent years, there is a need for further higher quality of image characteristics, so that the decrease of the grain diameters of the developer is being advanced. The decrease of grain diameters of the developer improves the quality of image on one hand while tending to increase rubbing force by the blade on the other hand. This increase of rubbing force causes the developer (toner) to slip through the cleaning blade because of chatter or the like of the cleaning blade and this slipping of the developer may cause a black-line-like cleaning failure.
In addition, when the friction resistance is high, friction heat will rise between the light receiving member and the cleaning blade to raise the temperature, and this friction heat may cause a fusion phenomenon in which the developer used for thermal fixation firmly adheres to the surface of the light receiving member. Particularly, this fusion phenomenon becomes more prominent in proportion to the decrease of grain diameters of the developer; in the first stage the fusion phenomenon is too weak to affect the image; but repetitive use makes seeds of small fusion, gradually grows them and at last causes black-line-like image defects.
As the solutions to such circumstances, there are required the measures including a method of increasing the urging pressure of the cleaning blade, a method of increasing the hardness of the elastic rubber blade to increase the rubbing force in order to increase the force for scraping off the developer attached to the surface of the light receiving member, and so on. Increasing the hardness of the blade changes the property of the blade from a rubber-like state to a glass state and thus makes the material fragile, so as to shorten the lifetime of the blade. Further, the above methods tend to increase the frictional force against the surface of the light receiving member, so that there are some cases in which the uneven shaving of the surface layer is rather promoted.
Further, there are sometimes used the methods of providing means for rubbing the light receiving member surface to effectively remove the ozone products including the method of using the roller charging or transfer in which a conductive rubber roller is in contact with the light receiving member surface while being applied with a voltage to reduce the ozone amount and to effect rubbing, the method of providing an elastic rubber roller or magnetic roller in the cleaner in the cleaning step to recover the remaining toner and to rub the light receiving member surface, or the like method.
However, also in this case, successively repeating the steps of charging, exposure, developing, transfer, separation, and cleaning may change the surface property of the light receiving member surface to gradually increase the wear resistance thereof. The increase of the wear resistance of the light receiving member surface similarly promotes the degradation of the cleaning blade to lower the cleaning property for the remaining toner thus causing cleaning failure.
Further, the increase of the wear resistance of the light receiving member surface may promote the degradation of the elastic rubber roller used in roller charging, roller transfer, cleaning roller, and so on to cause cleaning blade to lower the cleaning property for the remaining toner thus causing charging failure, transfer failure or cleaning failure.
Further, when the elastic rubber roller for rubbing the light receiving member surface is degraded, there is generated a difference in the rubbing force to sometimes cause uneven scraping of the light receiving member surface. When such uneven scraping is caused, the sensitivity of the electrophotographic characteristics becomes nonuniform to cause density irregularities in the image.
This phenomenon becomes more prominent particularly as the grain diameters of the developer decrease. However, as described above, in recent years, there is a need for further higher quality of image characteristics, so that the decrease of the grain diameters of the developer is now being advanced.
As a countermeasure against the uneven scraping by a blade, fusion, etc. as described above, there has hitherto been sometimes employed a method of providing a magnet roller or a cleaning roller of urethane rubber, silicone rubber, or the like to uniformly spread the developer to reach the cleaning blade, thereby relaxing retention irregularities of the toner on the blade surface.
However, because the magnet roller is somewhat inferior in rubbing force to the elastic rubber roller, there is a case where the image smearing or the toner fused on the light receiving member surface can be removed only partly depending on the conditions such as the surface properties of the light receiving member, the electrophotographic process used, the service environment, or the like, thus failing to sufficiently exhibit the rubbing effect.
Incidentally, in recent years, the tendency to personal use of copying machines and printers requires the important subjects of size reduction, cost reduction, and less need for maintenance of the electrophotographic apparatuses, so that in terms of further energy saving and ecology, the apparatus is also desirably designed without provision of the means for directly or indirectly heating the light receiving member.
Under such circumstances, there are needs for the light receiving member that does not cause the image smearing without provision of the heating means and needs for the electrophotographic apparatus that does not cause uneven scraping and that can stably supply high image quality without density irregularities or fusion for a long term under any electrophotographic process conditions.